1. Field of the Invention
The present invention relates to an improved humidity sensor.
In order to maintain the transparency and good visibility characteristics of plate glass, glass plates have been provided with heating devices which prevent the accumulation of ice, frost, condensed moisture, etc. on the plates thereby permitting unimpaired vision through the glass plates. The heating devices normally are a conductive film or electric resistance strips located on the surface of the glass plate or between laminated glass plates. These glass plates have been used in automobiles, aeroplanes, ships, and other vehicles; in window glass plates for buildings or in other various apparatus and the like which should provide clear visibility under all weather conditions. Recently, an improved glass plate defogger has been developed which prevents cloudiness on glass surfaces by automatically heating the same only when fog forms on the surface of the glass plate. Such a defogging glass plate comprises a pair of electrodes disposed on the surface of a glass plate as a moisture detecting terminal. When moisture condenses between the pair of electrodes, a change of current or impedance between the electrodes caused by current is detected. When the changing current or impedance reaches a predetermined value, the electric heating element is activated thus providing heat to the glass plate. When the electrodes of the conventional humidity sensor for detecting condensed moisture are used, the impedance between the electrodes under the influence of condensed moisture varies depending upon the surface conditions of the glass plate between the electrodes, the atmosphere over the electrodes and lapses in time. It is difficult to prepare a humidity sensor for detecting condensed moisture which maintain the same impedance over long periods of time. For example, a humidity sensor comprising electrodes made of RuO.sub.2 is chemically stable, but the impedance between the electrodes drifts upon ageing which adversely affects activation of the relay.
A need, therefore, continues to exist for an improved electrode sensing system for humidity sensors.